This invention relates to communication systems for transportation undertakings, such as mass transit and other railroad undertakings, operating vehicles along one or more than one predetermined route.
It is desirable to provide communication facilities in one or both directions between the vehicles of such a system and a fixed control centre, for the supply of operational data to the control centre and/or the provision of information or instructions to the crew of the vehicle and/or for remote or automatic control of the vehicle.
Conventional radio links are generally inadequate for these purposes, because many high-powered fixed transmitter/receiver stations would be required to ensure complete coverage of the route, and when the route is wholly or extensively in tunnel they may be practically impossible to operate.
Recent years have seen the adoption by the more advanced mass-transit undertakings of radiating cable systems in which a special type of high-frequency cable (a radiating cable) is used to transmit signals, at a frequency generally in the range 30-460 MHz, along the route and at the same time to couple them with an external radiation field that is confined to the immediate vicinity of the radiating cable and accessible to a relatively moveable antenna.
Usually the radiating cable is stationary and the antenna mounted on the vehicle, but proposals have been made to support a radiating cable along the length of a train for communication with a stationary transmitter and/or receiver.
Radiating cables are usually of coaxial construction with a "leaky" outer conductor either in the form of a wire braid or with at least one continuous slot and/or at least one row of mutually spaced apertures extending lengthwise along the cable.
The design of the radiating cable in such a system is a delicate balancing operation between transmission and radiation characteristics, since quite minor changes in attenuation or other characteristics can result in signals on the one hand being transmitted substantially entirely within the cable without adequate coupling to the external radiation field or on the other hand in radiation from the cable becoming so efficient that substantially all the signal is radiated within a short distance and none reaches the more distant parts of the route.
Radiating cable communication systems are of considerably value in the normal operation of the transportation system, and can have special benefits in case of mechanical breakdown or other mishap, provided that the radiating cable system itself continues to function.
On the other hand, especially in the kind of fail-safe automatic control system in which absence of a pilot signal is effective to stop trains, failure of the radiating cable communication system can cause passengers to be trapped temporarily in conditions of discomfort and possible danger, for example where the failure has been caused by a minor fire which has also resulted in smoke in the vicinity. There are also instances when communication between a control center and a train may be lost if a cable fails in a fire beyond the position of the train. It would therefore be desirable to use radiating cables capable of functioning at least for a short period (say at least ten minutes) under fire conditions. It is also desirable that the radiating cable itself should not contribute, or should contribute as little as possible, to the hazards arising under fire conditions, whether by spreading combustion or by producing smoke or fumes. Prior to my invention, cables satisfying these requirements were not available.